Effective diffuser/thickener screen backflushing

ABSTRACT

The screens of a pulp treating vessel, such as a paper pulp thickener, or diffuser, are back flushed in a simple yet effective manner. The screens are mounted to a supporting conduit within a vertical vessel, and a cylinder is connected at one open end to the conduit, and the other open end is disposed within the pulp inside the vessel. A piston is mounted within the cylinder and relative movement between the piston and cylinder is effected periodically in order to force liquid from within the cylinder into the conduit and out the screens, backflushing the screens and preventing clogging. Since no additional volume of liquid is added to the vessel during backflushing, no surge occurs in the vessel. Conventional accumulators, on/off valves, and the like can be eliminated while--in the case of diffusion washing--efficiency can be increased as much as 20-30%, and capacity increased as much as about 15%.

BACKGROUND AND SUMMARY OF THE INVENTION

There are a number of paper pulp treating operations in which screensmounted on a reciprocal supporting conduit are disposed in a vessel.Thickening, displacement bleaching, and diffusion washing are threetypical types of operations which are accomplished utilizing apparatusas typified by U.S. Pat. Nos. 3,704,603, 3,760,948, and 4,172,037 (thedisclosures of which are incorporated by reference herein). While suchtechniques are imminently effective for treating pulp, they are notwithout their drawbacks.

One drawback that limits the versatility of such techniques is the factthat in many installations, in order to prevent the screens fromclogging a volume of backflushing liquid is periodically introduced intothe vessels. This causes a surge in the vessel volume which isunacceptable in multi-stage treatment techniques. Further, the equipmentnecessary to effectively perform the backflushing is expensive andcomplicated. Typically, an accumulator (which would be of titanium andthus very expensive, in the case of displacement bleaching), small pumpfor filling the accumulator, two on-off valves, and a controller for thevalves, are necessarily provided in prior art systems. Even so, thebackflushing liquid is supplied to the vessel at less than optimumconditions since the accumulator typically supplies the backflushingliquid at a velocity that decreases from the start of application to thecompletion. Further, the constant opening and closing of the extractionand backflushing valve, and the resulting disruption on the treatmentprocess, restricts the efficiency and capacity of the treatment.

According to the present invention, the above-mentioned drawbacks areovercome in a simple yet effective manner. According to the presentinvention, the backflushing liquid is supplied from within the vesselitself, rather than from the exterior of the vessel. The backflushingliquid is typically provided, according to the invention, from acylinder which is mounted to the bottom of the screen supportingconduit. The cylinder has a first open end in communication with theinterior of the screen support conduit, and a second open end in opencommunication with the pulp being treated within the vessel. A piston ismounted within the cylinder, and relative movement between the pistonand cylinder is effected. Utilizing this apparatus, it is possible toentirely eliminate the accumulator and its associated pump, and toreplace the two on/off valves and their associated controller with asimple control valve. Thus extraction can be continuous rather thanintermittent, and the extraction volume is proportioned with respect tothe effective volume of the cylinder so that efficient backflushingoccurs.

Utilizing the invention not only is it possible to eliminate theaccumulator, piping, and its other associated components as describedabove, but it is also possible to more efficiently treat a largercapacity of pulp. For example, according to the invention it is possibleto increase capacity of pulp being treated by as much as about 15% inthe case of diffusion washing, while increasing the efficiency of thediffusion washing by as much as 20 to 30%. Since the entire volume ofbackflushing liquid is provided from within the vessel itself, surge iseliminated thereby expanding the number of treatment techniques that canbe practiced utilizing the invention.

Also according to the invention, a method of effecting treatment of pulpwith screens while minimizing screen clogging is provided, utilizing agenerally upright vessel having a pulp inlet and a pulp outlet, thescreens mounted to a supporting conduit within the vessel, and thescreens having screening surfaces extending generally vertically. Themethod comprises the steps of (a) Passing pulp generally vertically inthe vessel from the inlet to the outlet. (b) Effecting substantiallycontinuous extraction of liquid from the screens through the screensupport conduit at a predetermined flow rate. (c) Effecting generallyvertical movement of the screens and support conduit in the direction ofpulp low at a first speed, and in the direction opposite pulp flow at asecond speed much higher than the first speed. (d) When moving thescreens in the direction opposite pulp flow, supplying a known quantityof liquid to the conduit and screens from within the vessel to effectbackflushing of the screens without a surge in the vessel volume. And,(e) controlling the continuous extraction flow rate with respect to theknown quantity of backflushing liquid so that the known quantity ofbackflushing liquid is sufficient to effectively backflush the screensto prevent clogging thereof. The method according to the invention isparticularly adapted to (although not restricted to) diffusion washing,thickening, and displacement bleaching.

A significant object according to the present invention is toeffectively back flush screens in a pulp treatment vessel in a simplemanner. This and other objects of the invention will become clear froman inspection of the detailed description of the invention, and from theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view, partly in cross-section and partly in elevation,of an exemplary prior art diffusion washer with backflushing;

FIG. 2 is a side view, partly in cross-section and partly in elevation,of an exemplary diffusion washer according to the present invention;

FIG. 3 is a detail side cross-sectional view of the backflushingassembly of FIG. 2;

FIG. 4 is a cross-sectional detail view taken along lines 4--4 of FIG.3;

FIG. 5 is an enlarged side view, partly in cross-section and partly inelevation, taken along lines 5--5 of FIG. 2; and

FIG. 6 is a view like that of FIG. 3 only showing the zero position ofthe piston when a stationary piston is utilized.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a conventional diffusion washer which is used forwashing paper pulp, and effects backflushing of the screens to keep themunclogged. This conventional apparatus, indicated generally by referencenumeral 10, includes a generally upright vessel 11 having a pulp inlet12 and a pulp outlet 13 which are vertically spaced from each other. Thepulp is introduced into the pulp inlet 12, which typically is at thebottom, and flows vertically within the vessel 11, being dischargedthrough the conduit 13 adjacent the top. The basic operative componentsof the device 10 comprise a plurality of concentric cylindrical screens14, having screen faces 15 which are generally vertical and are incontact with the pulp. The concentric screens 14 are supported onsupport conduits 16, which typically are in the shape of arms extendingradially outwardly from a central hub section 17. The conduits 16 arehollow, and the interior may be divided into chambers if desirable.

The screen arms 16 are mounted for reciprocation in the verticaldirections by a reciprocating power structure or structures, preferablythree or more hydraulic cylinders 18 which are equally spaced around theperiphery of the device 10. The cylinders 18 reciprocate the screens 14and support arms 16 so that they move very slowly n the direction ofpulp movement (arrows A in FIG. 1) until the end of the vertical strokeis reached, and then reverse and move opposite to the direction A. Whilethe relative speeds in direction A and opposite to direction A can bevaried, in many installations the movement in direction A is much slowerthan the movement in the direction opposite to A (i.e. downwardly in theexemplary embodiment illustrated). This serves to assist in keeping thescreens 14 relatively clog-free.

To further alleviate the possibility of screen clogging, many commercialinstallations also utilize accessory backflushing liquid. Normally, atleast through most of the length of the stroke in A, liquid which hasbeen screened from the pulp and passed through screens 14 into arms 16,is extracted through conduit 20 (about 1 min), and on/off valve 21, andcarried to a remote site. When screen cleaning is desired controller 22closes the on/off valve 11, and opens the on/off valve 23. The valve 23is connected to an accumulator 24, which is recharged by a pump 25. Whenthe valve 23 is opened and the accumulator 24 is discharged (5-12 sec),liquid flows into the vessel 10 through the conduit 16 and moves fromthe interior of the screens 14 through the perforated faces 15 thereof,cleaning the perforations in the faces 15.

What has just been described above is utilizable for thickeningoperations. Where the pulp is to be treated with a liquid, too, such asa wash liquid or a bleaching liquid, then the rotating arms 30 withdepending vertical spray nozzles 31 are utilized, the arms 30 beingprovided with treatment liquid through the hollow shaft 32. Shaft 32 isrotated while supplying wash or bleaching liquid, or the like, by amotor 33.

There are a number of undesirable features associated with thestructures illustrated in FIG. 1. A significant difficulty is that whenback flush liquid is being supplied to the vessel 11 from theaccumulator 24, a "surge" takes place in the vessel 11 since theinterior volume of fluent material increases. Another difficulty isassociated with the constant switching on and switching off of theon/off valves 21, 23, with resultant time delays, the pulp treatment isnot as efficient as desirable, and which capacity is limited. Also, mostaccumulators typically discharge the liquid initially at a highpressure, and then at a lesser pressure as the gas driving the liquidout of the accumulator increases its volume. Also, the on/off valves 21,23, pump 25, and accumulator 24 (which must be made of expensivematerial such as titanium for some uses, such as displacement bleaching)are very expensive.

One exemplary embodiment of the invention is illustrated in FIGS. 2through 5, with portions of a second embodiment illustrated in FIG. 6.According to the invention all of the above-mentioned problemsassociated with the prior art have been eliminated in simple andeffective manner. Utilizing the structures such a illustrated in FIGS. 2through 5 for diffusion washing, for example, it is possible to increasecapacity by as much as about 15%, and to increase washing efficiency byas much as about 20 to 30%, compared to the conventional diffusionwashers illustrated in FIG. 1.

Component parts of the structure illustrated in FIGS. 2 through 5 thatare comparable to those in FIG. 1 are illustrated by the same referencenumeral only preceded by a "1".

The device 110 is very similar to the device 10, the major differencebeing the apparatus and manner for backflushing. Rather thanperiodically terminating extraction through conduit 120, in the device110 extraction is essentially continuous. The control valve (as opposedto an off/off valve) 40 is controlled to regulate the rate at whichliquid is being extracted from the pulp, which will depend upon whetheror not the pulp is being thickened or diluted or retaining the sameconsistency, the treatment liquid, etc. For the device 110, the sourceof backflushing liquid is located interiorly of the vessel 111 ratherthan exteriorly thereof. What this means is that when backflushingoccurs there is no surge within the vessel 111 since the volume offluent material within the vessel 111 remains essentially the same.

The backflushing apparatus, in the preferred embodiment illustrated inFIGS. 2 through 5, comprises a simple cylinder 42 which is open at bothends thereof. The first end 43 is open into the interior of the conduit116, and the second end 44 is open to the volume of pulp within thevessel 111. Mounted within the cylinder 42 for relative reciprocationwith respect to the cylinder 42 is a piston 46, which is connected to apiston rod 47. In the FIGS. 2 through 5 embodiment, that piston rod 47,which may be composed of several sections as illustrated by theconnector 48 (see FIGS. 2 and 5), is ultimately connected to a smallhydraulic cylinder 49 mounted atop the entire device 110. The cylinder49 effects movement of the piston 46 with respect to the cylinder 42.

The cylinder 42 and piston 46 need not be made of any special materials,and no particular care is necessary to see that there is a tight sealtherebetween. Small amounts of leakage between the piston 46 andcylinder 42 are inconsequential. It is only important to make sure thatthe piston 46 an cylinder 42 are of compatible material so that they donot harm each other.

FIGS. 3 and 4 illustrate in somewhat more detail than FIG. 2 one way inwhich the cylinder 42 can be attached to the hub section of the conduits116. These exact details of attachment are not particularly significantas far as the invention is concerned, however, it being necessary onlyto make sure that the connection is secure and that there is a free flowof extraction/backflushing liquid through the hole 50 in the bottom ofthe hub section of conduits 116, and the open first end 43 of thecylinder 42. Similarly, FIG. 5 illustrates the details of the topportion of the device 110 and shows how the piston rod 47 passes throughstuffing box 52, and through the conventional pipes 53, 54 which supplythe treatment fluid to the arms 130. Again, the details of theconnections are not particularly significant, only the fact that care betaken to minimize interruption of the flow of treatment liquid to thespray tubes 131 while still properly connecting the piston rod 47 to thecylinder 49. The cylinder 49 can be controlled from the same source ofhydraulic fluid as the cylinders 118.

FIG. 6 illustrates an embodiment in which the piston remains stationary.In this embodiment structures generally comparable to those in the FIGS.2 through 5 embodiment are illustrated by the same reference numeralonly preceded by a "2".

As can be seen in FIG. 6, the piston 246 is mounted stationary withrespect to the vessel 111 since the rod 247 is connected to a supportstructure 60 at the top of the device 110 in a relatively rigid manner.For example, only, in FIG. 6 cooperating nuts 61, 62 are shown whichengage a threaded end of the piston rod 247 and therefore hold it inplace. In this embodiment, the relative movement between the piston 246and the cylinder 242 takes place solely as a result of action by thecylinders 118, and the "zero" position of the components is asillustrated in FIG. 6, with the piston 246 adjacent the bottom, orsecond open end 244, of the cylinder 242.

Operation

With particular reference to FIGS. 3 through 4, an exemplary method ofeffecting backflushing during the treatment of the pulp will now bedescribed.

The pulp flows into inlet 112 and upwardly in direction A in vessel 111,ultimately being discharged through discharge 113. Treatment liquid,such as wash liquid or bleach liquid, is continuously added to the pulpvia the rotating arms 130 and the vertical spray tubes 131 dependingtherefrom. Liquid is withdrawn from the pulp through the screens 114,into the support conduits 116, passing out the conduit 120 through thecontrol valve 40. The cylinders 11 move the support conduits 116 and thescreens 114 slowly upwardly in the direction of pulp movement until theend of the stroke is reached, and then move them quickly downwardly atthe end of the upward stroke, to the bottommost position.

Just prior to the initiation of the downward stroke of the cylinders118, the cylinder 149 is actuated to quickly move the piston 46 upwardlywith respect to the cylinder 42. Although the exact speed of movement isnot particularly critical, typically the system would be designed sothat it took about two seconds for the piston 46 to move from itslowermost position (in which the volume within the cylinder 42 wasfilled with a maximum amount of backflushing/extraction liquid from theconduits 116 and with a minimum amount of pulp), to a more upwardposition (in which the cylinder 42 was filled with a minimum amount ofback flush/extraction liquid and a maximum amount of pulp).

The system is preferably designed so that the effective volume of thecylinder 42 (that is the amount of back flush/extraction liquid that itwill contain at the start of the backflushing operation) is enough toeffect efficient backflushing. Extraction continues during backflushing.In one typical utilization, for example, if the extraction flow ratethrough the conduit 120 controlled by the valve 40 is 6,000 liters perminute, the effective volume of the cylinder 42 will be between about10,000-12,000 liters, or roughly twice the amount of liquid extracted ina minute.

Of course as the piston 46 moves upwardly in the cylinder 42, the volumein the cylinder 42 behind the piston 46 is filled with pulp. Liquidwhich has been expelled from the cylinder 42 through open end 43 andhole 50, flows into conduit 116, through screens 114, and moves throughthe screen perforations, unclogging them.

After the backflushing operation, the cylinder 49 slowly moves thepiston 46 downwardly. As it does this, liquid from conduits 116 passesinto the cylinder 42 above the piston rod 46. This generally displacesthe pulp from behind the piston 46, as the piston 46 moves slowlydownwardly in the cylinder 42, and the cylinder 42 fills with liquidfrom conduits 116.

While the FIG. 6 embodiment is also effective in backflushing thescreens, it may be considered to be slightly less effective than theembodiment of FIGS. 2 through 5 since the backflushing action will notstart until there has been downward movement of the entireconduit/screen assembly 116, 114. Of course it is important that thelength of the cylinder 42, 242 be designed considering the stroke of thecylinders 118, and whether or not there will be an additional, separatestroke of the piston rod (the FIGS. 2 through 5 embodiment).

While the invention has been specifically described above with respectto thickeners, diffusion washers, and displacement bleachers, it is tobe understood that the invention also has other applications. Whereverno surge yet effective liquid movement within a pulp vessel is desired,the invention may be employed.

Further, it will be seen that the "backflushing" components according tothe invention can be considered to be a liquid pumping means in general,located within the vessel and including a chamber defining element(cylinder) and liquid engaging element (piston). While a piston andcylinder are the simplest and therefore the preferred form of suchcomponents, other equivalent fluid components, such as a rollingdiaphragm in a cylinder, etc., can be utilized where specificrequirements call for it. Thus the invention is to be accorded thebroadest interpretation of the appended claims so as to encompass allequivalent structures and methods.

What is claimed is:
 1. A pulp treating apparatus comprising:a generallyupright vessel defining an interior volume containing pulp to betreated; a pulp inlet to the vessel; a pulp outlet from the vessel, thepulp flowing generally vertically between the inlet and the outlet; aplurality of screens mounted within the vessel and connected tosupporting liquid conduits; extraction means for withdrawing liquid fromthe pulp, through the screens, and through the conduits to a pointoutside the vessel; and screen backflushing means mounted within thevessel and including a chamber defining element having first and secondends, the first end in open communication with liquid in the liquidconduits, and the second end in open communication with the pulp withinthe interior volume of the vessel; a liquid engaging element mountedwithin said chamber defining element for relative reciprocal movementbetween said liquid engaging element and said chamber defining element;and means for effecting relative movement between said chamber definingelement and said liquid engaging element to cause said liquid engaging,element to force liquid out of the chamber in one direction of relativemovement between said elements, to effect backflushing, and take liquidinto the chamber in another direction of relative movement therebetween.2. Apparatus as recited in claim 1 wherein said chamber defining elementcomprises an open-ended cylinder, and wherein said liquid engagingelement comprises a piston, and wherein said means for effectingrelative movement between said chamber defining element and said liquidengaging element includes a piston rod.
 3. Apparatus as recited in claim2 further comprising means for effecting vertical movement of saidscreens and supporting conduits in the direction of pulp movement at afirst speed, and in the direction opposite to pulp movement at a secondspeed much faster than said first speed.
 4. Apparatus as recited inclaim 3 wherein said means for effecting relative movement between saidpiston and cylinder consists essentially of said means for effectingvertical movement of said screens and supporting conduits, said pistonrod, and means for maintaining said piston rod stationary with respectto said vessel.
 5. Apparatus as recited in claim 3 wherein said meansfor effecting relative movement between said piston and said cylinderand said means for effecting vertical movement of said screens andsupport conduits are distinct structures.
 6. Apparatus as recited inclaim 5 wherein said means for effecting relative movement between saidpiston and cylinder includes a linear actuator mounted atop said vessel,and connected to said piston rod.
 7. Apparatus as recited in claim 3comprising a diffusion washer, and including moving spray tubes forintroducing wash liquid between said screens.
 8. Apparatus as recited inclaim 3 comprising a displacement bleacher, and including spray tubesfor introducing bleaching fluid between said screens.
 9. Apparatus asrecited in claim 1 further comprising means for effecting verticalmovement of said screens and supporting conduits in the direction ofpulp movement at a first speed, and in the direction opposite to pulpmovement at a second speed much faster than said first speed.
 10. Pulptreating apparatus comprising:a generally upright vessel defining aninterior volume containing pulp; liquid conduit means mounted in saidvessel; and liquid pumping means located within said vessel andcomprising a chamber defining element having first and second ends, saidfirst en in open communication with pulp in the interior volume of thevessel, and said second end in open communication with liquid in saidliquid conduit; a liquid engaging element mounted within said chamberdefining element for relative reciprocal movement between said liquidengaging element and said chamber defining element; and means foreffecting relative movement between said chamber defining element andsaid liquid engaging element to cause said liquid engaging element toforce liquid out of the chamber and into the liquid conduit means in onedirection of relative movement between said elements, and to take theliquid into the chamber from the liquid conduit means in anotherdirection of relative movement therebetween.
 11. Apparatus as recited inclaim 10 wherein said chamber defining element comprises an open endedcylinder, and said liquid engaging element comprises a piston, and saidmeans for effecting relative movement includes a piston rod. 12.Apparatus as recited in claim 11 further comprising screen meansdisposed in said vessel, the interior of said screen means operativelyconnected to said liquid conduit means.